Structure and selected properties of Al–Cr–Fe alloys with the presence of structurally complex alloy phases

The aim of the study was to supplement the data on the Al65Cr20Fe15 alloy with binary phase structure and the Al71Cr24Fe5 alloy with multiphase structure prepared with two different cooling rates from the liquid state. The presence of the structurally complex Al65Cr27Fe8 phase was confirmed by neutron diffraction, scanning electron microscopy with the analysis of chemical composition and transmission electron microscopy. Additionally, the Al8Cr5 phase with γ-brass structure was identified for Al71Cr24Fe5 alloy in both cooling rates from the liquid state. Due to the interesting features of structurally complex alloys, the wear resistance, magnetic properties, and corrosion products after performing electrochemical tests were examined. Based on pin-on-disc measurements, a lower friction coefficient was observed for the Al65Cr20Fe15 alloy (µ ≈ 0.55) compared to the Al71Cr24Fe5 multiphase alloy (µ ≈ 0.6). The average hardness of the binary phase Al65Cr20Fe5 alloy (HV0.1 = 917 ± 30) was higher compared to the multiphase Al71Cr24Fe5 alloy (HV0.1 = 728 ± 34) and the single phase Al–Cr–Fe alloys described in the literature. Moreover, the beneficial effect of rapid solidification on hardness was demonstrated. The alloys Al65Cr20Fe15 and Al71Cr24Fe5 showed paramagnetic behavior, however rapidly solidified Al71Cr24Fe5 alloy indicated an increase of magnetic properties. The studied alloys were characterized by the presence of passive layers after electrochemical tests. A higher amount of oxides on the surface of the Al71Cr24Fe5 alloy was recorded due to the positive effect of chromium on the stabilization of the passive layer.

www.nature.com/scientificreports/ as an approximant of quasicrystalline icosahedral and decagonal phases. In other publications 4,11 , the alloys Al 64.2 Cr 27.2 Fe 8.1 and Al 66.9 Cu 11.6 Fe 11.6 Cr 10.6 of structurally complex alloys were produced by hot sintering powders in the form of rolls with a diameter of 20 mm and then subjected to heat treatment. Based on the X-ray diffraction analysis, the Al 8 Cr 5 phase was identified for the Al 64.2 Cr 27.2 Fe 8.1 alloy and Al 6.5 Cr 0.5 Cu 2 Fe phase for the Al 66.9 Cu 11.6 Fe 11.6 Cr 10.6 alloy. The authors 11 concluded that the Al 8 Cr 5 phase (γ-brass) is isostructural with the Al 65 Cr 27 Fe 8 phase.
The purpose of the work was to provide detailed structural studies of the Al 65 Cr 20 Fe 15 and Al 71 Cr 24 Fe 5 alloys produced with two different cooling rates from the liquid state. In addition to the earlier work 20 , these alloys have not been described in terms of structure so far. Moreover, there is still a few experimental data that confirm the interesting properties of Al-Cr-Fe alloys with the CMAs, especially with binary and multiphase structure 4,5 . The results of the selected properties such as wear resistance, hardness, magnetic behavior, and chemical composition of the surface after corrosion were analyzed.

Materials and methods
Chemical elements of Al, Cr, and Fe with a purity of 99.99% were melted in an induction furnace with appropriate atomic fractions (Al 71 Cr 24 Fe 5 and Al 65 Cr 20 Fe 15 at.%) in a protective argon atmosphere in corundum crucibles (Φ = 30 mm, H = 45 mm) and then Ar-cooled. Ingots produced with a weight of 50 g were remelted and cast with an increased cooling rate from a liquid state under pressure (high-pressure die casting method with a cooling rate ~ 10 3 K/s) to a water-cooled copper mold (90 × 80 × 45 mm) in the form of plates (30 × 10 × 1 mm). X-ray diffraction, light microscopy observations, Mössbauer spectroscopy, differential scanning calorimetry, and electrochemical measurements were described for these alloys in an earlier publication 20 .
Neutron diffraction studies were performed on the MTEST neutron powder diffractometer at the Budapest Neutron Center. The Cu(111) monochromator was used which selected neutrons with a wavelength of λ = 0.1446 nm. The measured 2θ range was between 10° and 140°. This setup allowed a sufficient q-range and resolution for the identification of different phases present in the samples.
Observations of an ingot structure were made using scanning electron microscopy in backscattered electron (BSE) mode (Supra 35, Carl Zeiss) with EDX analysis to identify maps with the chemical composition of phases.
High resolution transmission electron microscopy (HRTEM) was used to determine electron diffraction from the selected area (SAED), structure, and morphology using S/TEM TITAN 80-300. Samples for HRTEM observations were powdered.
Coercive force (H c ) and saturation magnetization (M s ) were determined from changes of magnetization as a function of the magnetic fields up to 10 kOe. Magnetic properties were recorded using a LakeShore 7307 vibrating sample magnetometer.
Tribological tests were performed using the pin-on-disc method using CSM Instruments. The experiments were carried out on cylindrical ingots with a radius of 26 mm and a height of 3 mm. The radius of the wear track was 8 mm. A ball made of 100C6 steel with a diameter of 6 mm was used as a counter-sample. The linear speed was 0.01 m/s and a load of 10 N was applied. Observations of the wear tracks, together with measurements of their width after tribological tests, were carried out by scanning electron microscope (Supra 35, Carl Zeiss). Hardness tests were performed using a Future Tech FM-700 Vickers hardness testing instrument with a load of 100 g for 15 s.
The corrosion products on the surface of the Al 65 Cr 20 Fe 15 and Al 71 Cr 24 Fe 5 samples in the form of plates after corrosion tests in 3.5% NaCl solution at 25 °C were determined by X-ray photoelectron spectroscopy (XPS). Depth profile mode (DP-XPS) using a Physical Electronics (PHI 5700/660) spectrometer working under an ultrahigh vacuum (10 −9 Torr) in UHV cluster and a monochromatic Al Kα X-ray source (1486.6 eV) was used. Both tested samples were initially kept pre-chamber held under vacuum (10 −8 Torr) for at least 1 h, next transferred to the measurement chamber and analyzed. The survey spectra were measured with a pass energy of 187.85 eV. Depth profile (DP-XPS) analysis was carried out using a focused 1.5 kV Ar + beam for 15 min, sputtering in intervals between measurements. Core-level lines collected in the DP-XPS analysis were measured with a pass energy of 23.5 eV. All obtained XPS data was analyzed using MultiPak 9.7 software, which contains an internal reference database and compared to the NIST XPS database.
Ethical approval. This article does not contain any studies with human participants or animals performed by any of the authors.

Results and discussion
Based on the phase analysis provided by the XRD method presented in 20  . The presence of the identified phases in the structure for the ingots was confirmed by the analysis of the neutron diffractograms in Fig. 1. The presence of the Fe 2 CrAl and Cr phases was excluded for Al 71 Cr 24 Fe 5 due to the small number of matched reflections. Furthermore, observations were carried out using scanning electron microscopy in BSE mode with the EDX analysis presented in Fig. 2 for Al 65 Cr 20 Fe 15    www.nature.com/scientificreports/ Al-Cr-Fe alloys were described in the literature primarily in terms of the formation of structurally complex phases. Ura-Binczyk et al. 4,11 studied a polycrystalline alloy of Al 64.2 Cr 27.2 Fe 8.1 , which was produced by hot press sintering of the intermetallic powders and heat treated. The authors 11 pointed out that γ-Al 8 Cr 5 phase identified by XRD is isostructural with Al 65 Cr 27 Fe 5 . In this study, the Al 65 Cr 27 Fe 6 phase had lattice parameters a = b = 12.6963 and c = 7.9211 and the angles between them α = β = 90° and γ = 120° in the hexagonal notation, which is consistent with the data reported in the articles 11,16 . According to report presented by Veys et al. 19 the phase of Al 65 Cr 27 Fe 8 has γ-brass structure and is isostructural to cubic Al 9 Cr 4 (with lattice parameters a = 9.4 Å).
In this work, for the multiphase Al 71 Cr 24 Fe 5 alloy, the Al 8 Cr 5 phase was also identified for both cooling rates. The lattice parameters of which a = b = c = 7.8050 and α = β = γ = 109.127° correspond to the α-Al 8 Cr 5 phase, which corresponds to the rhombohedral system. The parameters of the unitary lattice cell are consistent with the description 12 . According to 10,12,21 , the Al 8 Cr 5 phase with a rhombohedral structure has a γ-brass structure.
Additionally, the results were compared with a study in which Al-Cr-Fe based alloys were cast 13 . Two phases were marked in the SEM image for the Al 66 Fe 22 Cr 12 alloy: α-Al 8 Cr 5 and Al 5 Fe 2 . The microstructure is similar to that shown in Fig. 2 for Al 65 Cr 20 Fe 15 13 . Many researchers study the surface properties of complex metallic alloys because of the specific electronic structure associated with high symmetry clusters and unit cells made of thousands of atoms. Quasicrystals, which are included in the group of complex metallic alloys, are characterized by a low coefficient of friction and high wear resistance 2 . To describe abrasion resistance, tribological tests using the pin-on-disc method were performed. The test measurements with the parameters described in article 22 were carried out, however, no clearly signs of wear were observed, due to the low linear speed 0.05 m/s and the distance 8 m as well as relatively low load (F N = 2 N). We used the parameters described in 23 . Figure 6 presents a graph of the dependence of the friction coefficient on the distance, which was recorded during the pin-on-disc for Al 65 Cr 20 Fe 15  The results of pin-on-disc tests were also described in 24 , which compared the Al-Cu-Fe-Cr and Al-Cu-Fe alloys used for the coatings. It could be compared that for the chemical composition with the addition of chromium, the friction coefficient was similar to the results described in this article for Al 65 Cr 20 Fe 15 25,26 noted that the characteristic friction values are lower than in the air atmosphere. This is because the oxide layer has a significant influence on the measured value of the friction coefficient. Taking into account the fact that the alloys described in our work are binary and multiphase, it could be assumed that the wear resistance is similar to the single phase alloys 25,26 . Figure 7 shows the morphology of the wear tracks studied by SEM. It could be observed that three types of wear mechanisms dominated the trace of formation: plastic deformation, delamination, and oxidation. The identification of wear mechanisms was supported by the results described in paper 27 . Duckham et al. 28 investigated the wear resistance of quasicrystalline Al-Pd-Mn and Al-Ni-Co alloys. Wear tracks were also observed after pin-on-disc tests using microscopic methods. The authors 28 paid attention to the characteristic cracks that also appeared for the Al 65 Cr 20 Fe 15 and Al 71 Cr 24 Fe 5 alloys studied. This mechanism is called by 28 as the ring cracks, characteristic of brittle materials, which indicates the maximum tensile stress. The article 28 also describes the partial removal of the material, which is a delamination. The publications of Dubois et al. 25,26 described the phenomenon of oxidation during pin-on-disc tests caused by the air atmosphere, which was observed using SEM in the form of oxide debris. Furthermore, wear track width measurements were carried out, the mean values of which were 1.23 (± 0.05) and 1.   Table 1. In numerous studies, the influence of the structure on the magnetic properties was observed. In the case of the Al 65 Cr 20 Fe 15 alloy, the saturation magnetization was higher for the ingot form. The relationship was opposite and a lower value was noted for the ingot of the Al 71 Cr 24 Fe 5 composition. The coercivity was several times higher for the plates in   www.nature.com/scientificreports/ both chemical compositions. It may be related to changes in the structure under the influence of the cooling rate from the liquid state. In work 34 for Fe-based alloys with nanocrystalline structure, changes in coercivity resulting from grain growth after annealing were observed. In this work, the opposite phenomenon was observed because increasing casting conditions lead to fragmentation of the structure. The alloys studied showed paramagnetic properties. On the basis of the obtained results, a decrease of magnetic properties is visible along with an increase in the cooling rate from the liquid state for the Al 65 Cr 20 Fe 15 alloy and an increase for the Al 71 Cr 24 Fe 5 alloy. Paramagnetic properties were also described in 2,33 for the single phase Al 80 Cr 15 Fe 5 SCAP-type alloy. Furthermore, polycrystalline, structurally complex Al 86 Cr 6 Fe 6 31 and Al 61.3 Cr 31.1 Fe 7.6 32 alloys were previously described in the literature as paramagnets. Based on the research conducted, it could be concluded that the presence of crystalline phases in the Al 65 Cr 20 Fe 15 and Al 71 Cr 24 Fe 5 alloys did not change the magnetic properties at room temperature.
The corrosion resistance of Al-Cr-Fe alloys was reported in the paper 20 . Electrochemical measurements of the open circuit potential as a function of time and potentiodynamic polarization curves were recorded in a 3.5% NaCl aqueous solution at a temperature of 25 °C. Electrochemical impedance spectroscopy tests were also carried out. The electrochemical parameters, such as E OCP , E corr , R p , and j corr for studied Al-Cr-Fe alloys varied, which indicates differences in the corrosion mechanism. Among others, the Al 65 Cr 20 Fe 5 alloy in the form of plate showed a corrosion potential closer to the positive values, although a higher polarization resistance was observed for the Al 71 Cr 24 Fe 5 plate. Analysis of corrosion products is a useful tool to assess the corrosion behavior of materials; therefore, this article presents the results of the XPS analysis for Al 65 Cr 20 Fe 15 and Al 71 Cr 24 Fe 5 alloys in the form of plates after corrosion tests 35 .
The XPS survey spectra for the surface of Al 65 Cr 20 Fe 15 (a) and Al 71 Cr 24 Fe 5 (b) in the form of plates are presented in Fig. 10. The characteristic peaks (O1s, C1s, Al2s, Al2p, Cr2p, Cr3p) and the Auger spectrum (for O KLL and C KLL) were identified. High intensities relative to the binding energy of oxygen may indicate the formation of a passive layer in the tested plates. Figures 11 and 12 present the XPS core level lines of Al2p, Cr2p, Fe2p, O1s acquired during depth profile measurements for Al 65 Cr 20 Fe 15 and Al 71 Cr 24 Fe 5 plates, respectively. As one may notice at the surface, high binding energy Al2p and Cr2p peaks typical for oxides are evident. These results indicate the formation of a passive layer of Al 2 O 3 and Cr 2 O 3 . Along with removing successive atomic layers by the argon beam, Al2p and Cr2p are typical for pure aluminium and chromium elements. It is worth noting that for both plate samples, the Fe2p line is typical for metallic iron with a spin-orbit splitting of about ΔE ≈ 12.8 eV. The XPS depth profiles for plates Al 65 Cr 20 Fe 15 (a) and Al 71 Cr 24 Fe 5 (b) are shown in Fig. 13. As the sputtering time and depth increased, the samples analyzed showed a significantly lower percentage atomic concentration of C1s, indicating the presence of carbon impurity usually accumulated on the surface. In the case of O1s, the same tendency can be noticed. Oxygen in the initial stage of sputtering may indicate the presence of oxygen, as a typical impurity on the surface that overlaps with oxygen formed by the passive layers. As successive atomic        36 , it was found that the addition of chromium is necessary for the stabilization of the passive layer. Therefore, the Cr 2 O 3 was identified for the Al 71 Cr 24 Fe 5 alloy with a higher chromium content, which positively influences the corrosion resistance.

Conclusions
• Structural studies using ND, SEM-EDX, and TEM methods confirmed the presence of two phases for the Al 65 Cr 20 Fe 15 alloy and multiple phases for Al 71 Cr 24 Fe 5 . Both alloys were characterized by the presence of the structurally complex alloy phase-Al 65 Cr 27 Fe 8 . The Al 8 Cr 5 phase with γ-brass structure was identified for Al 71 Cr 24 Fe 5 alloy in a form of ingot and plate.

Data availability
The data and material generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.